DESCRIPTION

Geocentric Coordinate Time (TCG) is a coordinate time scale representing time in the Terran system. Specifically, it is the proper time experienced by a distant clock comoving with the geocentre. It is linearly related to Terrestrial Time (TT), which is the proper time scale underlying timekeeping on the Terran surface. TT is formally defined in terms of TCG: TT ticks exactly 0.999_999_999_303_070_986_6 seconds for each second of TCG.

This module represents instants on the TCG and TT time scales as scalar numbers of SI seconds since an epoch. This is an appropriate form for all manner of calculations. Both scales are defined with a well-known point at TAI instant 1977-01-01T00:00:00.0. This point is used as the epoch for TCG, having the scalar value zero. The same instant on the TT scale is assigned the scalar value 599_616_000 exactly, corresponding to an epoch near the TAI epoch 1958-01-01T00:00:00.0. This matches the convention used by Time::TT for instants on the TT scale. The use of very different epochs for the two scales avoids confusion between them.

There is also a conventional way to represent TCG instants using day-based notations associated with planetary rotation `time' scales. The `day' of TCG is a nominal period of exactly 86400 SI seconds, which is slightly shorter than an actual Terran day. The well-known point at TAI instant 1977-01-01T00:00:00.0 is assigned the label 1977-01-01T00:00:32.184 (MJD 43144.0003725). Because TCG is not connected to Terran rotation, and so has no inherent concept of a day, it is somewhat misleading to use such day-based notations. Conversion between this notation and the linear count of seconds is supported by this module. The day-based notations for TT and TCG instants yield very similar values for corresponding instants, so care must be taken to avoid confusion.

Because TCG is a theoretical time scale, not directly accessible for practical use, it must be realised using atomic clocks. In fact, it is TT that is directly so realised, but the linear relationship between TT and TCG means that any realisation of TT is effectively also realising TCG. This module supports conversion of times between different realisations of TCG, by making use of the facility in Time::TT for realisations of TT.

FUNCTIONS

tcg_instant_to_mjd(INSTANT)

Converts from a count of seconds to a Modified Julian Date in the manner conventional for TCG. The MJD can be further converted to other forms of day-based date using other modules. The input must be a Math::BigRat object, and the result is the same type.

tcg_mjd_to_instant(MJD)

Converts from a Modified Julian Date, interpreted in the manner conventional for TCG, to a count of seconds. The input must be a Math::BigRat object, and the result is the same type.

tcg_to_tt(TCG_INSTANT)

Converts from a TCG instant (as a count of seconds from the TCG epoch) to the corresponding TT instant (as a count of seconds from the TT epoch). The input must be a Math::BigRat object, and the result is the same type.

tt_to_tcg(TT_INSTANT)

Converts from a TT instant (as a count of seconds from the TT epoch) to the corresponding TCG instant (as a count of seconds from the TCG epoch). The input must be a Math::BigRat object, and the result is the same type.

tcg_realisation(NAME)

Looks up and returns an object representing a named realisation of TCG. The object returned is of the class Time::TCG::Realisation; see the documentation of that class for its interface. Each TCG realisation corresponds precisely to a realisation of TT. The realisation names that are understood are exactly the same as those understood by tt_realisation in Time::TT.